1. Field of the Invention
The present invention relates to a surface coil of the type suitable for obtaining nuclear magnetic resonance signals from an examination subject in a nuclear magnetic resonance examination apparatus.
2. Description of the Prior Art
Nuclear magnetic resonance examination devices may use a surface coil to obtain nuclear magnetic resonance signals from the examination subject to identify spectra or images of the subject. The nuclear magnetic resonance apparatus includes a set of coils for generating a fundamental magnetic field, and a set of coils for generating gradient magnetic fields, with the patient being disposed in both the fundamental field and the gradient fields. The examination subject is irradiated via an antenna with a sequence of radio frequency pulses, causing nuclear magnetic resonance signals to arise within the examination subject. These signals are acquired by the surface coil, which is connected to an evaluation unit through a tuning circuit. Such a surface coil is disclosed, for example, in published patent application WO 84/00 214.
Surface coils are used in nuclear magnetic resonance tomography and in nuclear magnetic resonance spectroscopy when specific parts of the examination subject must be interpreted with improved signal-to-noise ratio. Specifically, in spinal column diagnosis, a compromise must be made using conventional surface coils. Large (oval) surface coils enable a large measuring field to be examined with a high penetration depth. The signal-to-noise ratio for such a large area, however, is poor. Small (generally round) surface coils have a favorable signal-to-noise ratio but permit only a small measuring field and a slight penetration depth to be examined.
To cover a large region to be examined using small surface coils with a favorble signal-to-noise ratio, it is possible to successively displace the small surface coil relative to the examination subject. In the case of spinal column examinations, however, the surface coils are generally embedded in bed cushions, so that such displacement is extremely difficult in practice.
In practice, therefore, a critical region is first localized using a large surface coil. The patient is then shifted onto a bed cushion having a small, high-resolution surface coil. Such shifting requires re-adjustment of all critical parameters. This method is therefore time consuming, and is extremely uncomfortable for the patient, particularly for patients having spinal column injuries.